• Journal of the Society of Naval Architects of Korea
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• v.39 no.3
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• pp.28-40
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• 2002

Turbulent flows around two-dimensional wing sections in ground effect are analysed by incompressible RANS equations and a finite difference method. The Baldwin-Lomax algebraic turbulence model is used to simulate high Reynolds number flows. The main purpose of this study is to clarify the two-dimensional ground effect and its flow characteristics due to different ground boundary conditions, i.e., moving and fixed bottom boundary. As a first step, to validate the present numerical code, the computational result of Clark-Y(t/C 11.7%) is compared with published numerical results and experimental data. Then, NACA4412 section in ground effect is calculated for various ground clearances with two bottom boundary conditions. According to the computational results, the difference in the lift and moment simulated with the two bottom boundary conditions is negligible, but the drag force simulated by the fixed bottom is to some extent smaller than that by the moving bottom. Therefore, it can be concluded that the drag force measured in a wind tunnel with the fixed bottom could be smaller than that with the moving bottom.

• Journal of Ocean Engineering and Technology
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• v.35 no.5
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• pp.382-392
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• 2021

In recent years, floating offshore wind turbines have attracted more attention as a new renewable energy resource while bottom-fixed offshore wind turbines reach their limit of water depth. Various projects have been proposed with the rapid increase in installed floating wind power capacity, but the economic aspect remains as a biggest issue. To figure out sensible approaches for saving costs, a comparison analysis of the levelized cost of electricity (LCOE) between floating and bottom-fixed offshore wind turbines was carried out. The LCOE was reviewed from a social perspective and a cost breakdown and a literature review analysis were used to itemize the costs into its various components in each level of power plant and system integration. The results show that the highest proportion in capital expenditure of a floating offshore wind turbine results in the substructure part, which is the main difference from a bottom-fixed wind turbine. A floating offshore wind turbine was found to have several advantages over a bottom-fixed wind turbine. Although a similarity in operation and maintenance cost structure is revealed, a floating wind turbine still has the benefit of being able to be maintained at a seaport. After emphasizing the cost-reduction advantages of a floating wind turbine, its LCOE outlook is provided to give a brief overview in the following years. Finally, some estimated cost drivers, such as economics of scale, wind turbine rating, a floater with mooring system, and grid connection cost, are outlined as proposals for floating wind LCOE reduction.

• Journal of Welding and Joining
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• v.18 no.4
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• pp.28-37
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• 2000

In this paper the LNG vessel of the Moss type which is capable of lifting 15,261 tons is investigated in the view point of the pressure vessel preliminary design using the finite element method. The Pressure vessel design is based on the equivalent stress levels due to the internal pressure. The finite element model of the spherical pressure vessel is configured using 4 noded quadrilateral shell element. The finite element analysis program NASTRAN and ANSYS 5.5are implemented. The design is compared with the three kinds of the boundary condition : first, where the equator of the pressure vessel is fixed, and where the top and is fixed, and, the bottom end is fixed, respectively. A comparison is presented between the results obtained by the finite element model and by the prototype production model. Additionally just below position(case 1 & case 2) of equator ring was carried out by using ANSYS 5.5. The results show that the vessel design based on the stress is acceptable at the preliminary design.

• Wind and Structures
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• v.21 no.6
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• pp.625-639
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• 2015

Monopiles have been most widely used for supporting offshore wind turbines (OWTs) in shallow water areas. However, multi-member lattice-type structures such as jackets and tripods are also considered good alternatives to monopile foundations for relatively deep water areas with depth ranging from 25-50 m owing to their technical and economic feasibility. Moreover, jacket structures have been popular in the oil and gas industry for a long time. However, several unsolved technical issues still persist in the utilization of multi-member lattice-type supporting structures for OWTs; these problems include pile-soil-interaction (PSI) effects, realization of dynamically stable designs to avoid resonances, and quick and safe installation in remote areas. In this study, the effects of PSI on the dynamic properties of bottom-fixed OWTs, including monopile-, tripod- and jacket-supported OWTs, were investigated intensively. The tower and substructure were modeled using conventional beam elements with added mass, and pile foundations were modeled with beam and nonlinear spring elements. The effects of PSI on the dynamic properties of the structure were evaluated using Monte Carlo simulation considering the load amplitude, scouring depth, and the uncertainties in soil properties.

• Journal of the Korean Institute of Electrical and Electronic Material Engineers
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• v.33 no.4
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• pp.281-285
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• 2020

Oxide semiconductor devices have become increasingly important because of their high mobility and good uniformity. The channel length of oxide semiconductor thin film transistors (TFTs) also shrinks as the display resolution increases. It is well known that reducing the channel length of a TFT is detrimental to the current saturation because of drain-induced barrier lowering, as well as the movement of the pinch-off point. In an organic light-emitting diode (OLED), the lack of current saturation in the driving TFT creates a major problem in the control of OLED current. To obtain improved current saturation in short channels, we fabricated indium gallium zinc oxide (IGZO) TFTs with single gate and double gate structures, and evaluated the electrical characteristics of both devices. For the double gate structure, we connected the bottom gate electrode to the source electrode, so that the electric potential of the bottom gate was fixed to that of the source. We denote the double gate structure with the bottom gate fixed at the source potential as the BGFP (bottom gate with fixed potential) structure. For the BGFP TFT, the current saturation, as determined by the output characteristics, is better than that of the conventional single gate TFT. This is because the change in the source side potential barrier by the drain field has been suppressed.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.3 no.3
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• pp.197-205
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• 1991

A numerical study has been performed to investigate two dimensional natural convection heat transfer in a rectangular enclosure with heat sources of constant temperature at the bottom. Calculations were made for various dimensionless heat source lengths, W/L=0.1-0.5, and positions of heat sources at $Gr=2.57{\times}10^6$, Pr=0.71 and Ks/Kf=28.98. For various positions of heat sources, the maximum local Nusselt numbers generally show X=0.81-0.85 at the bottom and X=0.23 at the top. For various dimensionless heat source lengths, the maximum local Nusselt numbers at the bottom show W/L=0.4 for one heat source, W/L=0.2 for two heat sources with fixed centers, W/L=0.5 for two heat sources with moved centers. Finally the maximum heat transfer at the bottom exhibits in condition of W/L=0.4 for two heat sources with moved centers.

• Journal of Korean Port Research
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• v.7 no.1
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• pp.79-88
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• 1993

This study deals with the case of a fixed floating structure(FFS) at the mouth of a rectangular harbor under the action of waves represented by the linear wave theory. Modified forms of the mild-slope equation is applied to the propagation of regular wave over constant water depth. The model is extended to include bottom friction and boundary absorption. A hybrid element approximation is used for calculation of linear wave oscillation in and near coastal harbor. Modification of the model was necessary for the FFS. For the conditions tested, the results of laboratory experiments by Ippen and Goda(1963), and Lee (1969) are compared with the calculated one from this model. The cases of flat cylinderical structures, both fixed and floating, were taken to be in an intermediate water depth.

• Journal of the Korean Institute of Electrical and Electronic Material Engineers
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• v.31 no.7
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• pp.481-485
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• 2018

In this study, oxide/metal/oxide-type transparent electrodes based on Al and ZnO were investigated. Thin films of these materials were sputter-deposited at room temperature. To evaluate the thickness dependence of the oxide layers, the top and bottom ZnO layers were varied in the range of 5~80 nm and 2.5~20 nm, respectively. When the thicknesses of the top and bottom ZnO layers were fixed at 30 nm and 2.5 nm, a maximum transmitance of 66% and sheet resistance of $16.5{\Omega}/{\square}$ were achieved, which is significantly improved compared with the Al layer without top and bottom ZnO layers showing a maximum transmitance of 44.3% and sheet resistance of $44{\Omega}/{\square}$.

• Magazine of the Korean Society of Agricultural Engineers
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• v.30 no.2
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• pp.44-54
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• 1988

This study was carried out to investigate the applicability of the conventional design method for ground supported circular cylindrical shell structures. For this purpose, the ensiled farm silo was adopted as a model structures. Herein, the conventional design method was based on the assumption that such structures are clamped at the bottom edges or the ground pressure is independent of the deflection at the surface. In the present paper, the applicability of above assumption was checked out by comparison with an exact method considering soil-structure interaction. Some results of numerical calculation show us ; When the ground is very hard, for example Winkler's constant k is larger than 100 kg / cm$^2$ / cm, or the bottom plate of structures has a infinitely stiffness, for example the bottom plate thickness is larger than 100 cm, the sectional forces, obtained from the conventional method at any wall of structures resting on an elastic foundation, can used for design purpose. Therefore, if the above condition is satisfied then the conventional assumptions can be justified for the design purpose. In this case, the assumption that such structures are fixed at the lower edges was more realistic than the assumption that the reaction pressure acting on structures is uniformly disributed since the accuracy of results of the analysis by the former assumption was higher than that obtained from the latter assumption. But the sectional forces in the bottom plate resting on ground directly could not be evaluate correctly by the conventional method.

• The Journal of the Acoustical Society of Korea
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• v.15 no.4
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• pp.18-23
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• 1996

Fluctuation statistics of scattering strength are not only important because they impact the performance of active sonar systems, but also because they may provide insight into the major scattering process. In this article, analysis of the statistical characteristics of bottom backscattering, measured in shallow water, are presented. The slowly moving experimental sonar was operated at 30kHz to gather data over the bottom. Spatial and temporal correlation functions of the signal amplitudes were measured. The distribution function and probability of false alarm function of the detected envelope of widebeam and narrowbeam signals were measured. An attempt was made to compare the results with existing theoretical models. The result suggests that the statistical characteristics of bottom backscattering fluctuation of moving source is differ from that of fixed source.